1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a printer or a facsimile apparatus, provided with a cleaning apparatus in which a cleaning blade is maintained in contact with an image bearing member such as an image bearing member or an electrostatic recording dielectric member (hereinafter also called a photosensitive member) to remove used toner on the image bearing member.
2. Related Background Art
In the image forming apparatus of this kind, an electrostatic latent image formed on the surface of a running image bearing member is developed as a toner image by developing means, and such toner image is transferred onto a transfer material, while the toner which is not transferred onto the transfer material but remains on the surface of the image bearing member is cleaned and removed by the cleaning apparatus. In such cleaning apparatus, a cleaning blade formed by an elastic member such as of polyurethane rubber is commonly used widely in the related art as the cleaning means for the remaining toner, and such cleaning blade is used because it is simple and compact in structure and also advantageous in cost.
Recently, for the purpose of compactization of the apparatus and reducing the discharge time of a first print, the image forming apparatus is employing a configuration in which a recording sheet (transfer sheet) is conveyed vertically (in the direction of gravity) from below to above. In such image forming apparatus, an aperture of the cleaning apparatus is directed downwards.
When an image bearing member 72 is rotated in a direction indicated by an arrow R, the surface of the image bearing member 72 is uniformly charged by a charger 73. The charged portion is subjected to an optical image exposure by an optical signal generating apparatus 74 to form an electrostatic latent image. The electrostatic latent image, upon passing a developing apparatus 75, is developed into a visible image with developing toner. The toner image thus formed is transferred by transfer means 79 onto a transfer paper (hereinafter also called transfer material) P, and the transfer paper P after the transfer is separated from the image bearing member 72 and, after fixing in fixing means, discharged onto a discharge tray 94 by paired discharge rollers 82.
The untransferred toner remains on the surface of the image bearing member 72 after the transfer, and such toner T is scraped off by a cleaning blade 84 of a cleaning apparatus 83 in contact with the surface of the image bearing member 72. A scooping sheet 86 is provided in order to prevent leakage of the toner T from the cleaning unit.
In the above-mentioned cleaning apparatus 83, because a cleaning container has a downward aperture, the toner T tends to stay by the gravity, in a portion formed by the image bearing member 72, the cleaning blade 84 and the scooping sheet 86. The cleaning apparatus 83 is provided with a screw 103 in order to discharge such stagnant toner, but the amount of the stagnant toner exceeds a toner discharging ability of the screw 103 in case sheets with a high print ratio are passed in a large amount. Therefore, in case the toner T continues, to stay in such state, there will result a situation where not only a satisfactory cleaning cannot be achieved but also the toner T overflows from the cleaning apparatus.
Also a distance from a contact position between the cleaning blade 84 and the image bearing member 74 to a contact position between the scooping sheet 86 and the image bearing member 74, namely an aperture between the front end of the cleaning blade and the front end of the scooping sheet, has to be finely set at about 1 mm in order to prevent toner leakage. In order to meet these requirements, it is necessary to improve a transfer rate at the transfer of the toner image from the image bearing member to the transfer material P.
Japanese Patent Application Laid-open No. 9-26672 discloses a method of reducing a volumic resistivity of the toner by including a transfer efficiency improving agent with an average particle size of 0.1 to 3 xcexcm and hydrophobic fine silica powder of a BET specific surface area of 50 to 300 m2/g by a nitrogen adsorption method and improving the transfer efficiency by forming a thin film of the transfer efficiency improving agent on the photosensitive member. However, since the toner prepared by a crushing method has a particle size distribution, it is difficult to obtain an effect in all the particles and there is required a further improvement.
In order to obtain a toner shape closer to spherical as a method of improving the transfer efficiency, toner produced by a spray particle-forming method, a solution dissolving method, a polymerization method etc. is disclosed for example in Japanese Patent Application Laid-open No. 3-84558, Japanese Patent Application Laid-open No. 3-229268, Japanese Patent Application Laid-open No. 4-1766, Japanese Patent Application Laid-open No. 4-102863, etc. However, such toner producing methods not only require a large facility but also involve drawbacks on the cleaning because of a fact that the toner particles become closer to true spheres, and cannot be considered preferable for the sole purpose of improving the transferability.
The toner is generally formed by employing as raw materials, a binder resin for fixation to the transfer material, a colorant for forming a color in the toner, and a charge controlling agent for providing the particle with a charge, and, in case of so-called one-component developing method as disclosed in Japanese Patent Application Laid-open No. 54-42141 and Japanese Patent Application Laid-open No. 55-18656, additionally employing a magnetic material for providing the toner with a carrying property. Such materials are dry mixed together with other additives such as a releasing agent or a fluidity providing agent if necessary, then fused and kneaded in a general-purpose kneading apparatus such as a roll mill or an extruder, and, after solidification by cooling, the kneaded substance is crushed by a crushing apparatus such as jet flow crusher or a mechanical collision crusher. The obtained crude crushed substance is introduced into an air classifier for classification thereby providing a classified material with a particle size required as toner, which is dry mixed with an external additive such as a fluidizer or a lubricant if necessary to obtain toner usable for image formation.
In case of the toner to be used in a two-component developing method, the above-mentioned toner is mixed with magnetic carrier for use in the image formation. There is desired toner of a high developability with an improved transfer efficiency, for the purpose of reducing the toner remaining on the photosensitive member after the transfer and constituting the waste toner, including the productivity of the toner itself.
An object of the present invention is to provide an image forming apparatus capable of effectively preventing generation of defective cleaning, also in case of employing cleaning means in which a container has a substantially downward aperture in the direction of the gravity.
Another object of the present invention is to provide an image forming apparatus including:
image forming means which forms a developer image on a rotatable image bearing member;
transfer means which transfer the developer image on the image bearing member onto a transfer material; and
cleaning means having a cleaning member maintained in contact with the image bearing member and a container of which an aperture for recovering the developer is substantially downward in the direction of gravity;
wherein the developer has a substantially spherical shape.